The present invention relates to the production of metallic elements by fluid pressure deformation, or hydroforming, and more particularly to the production, by this technique, of sheet metal.
Hydroforming is a well-known technique which permits, in one of its principal modes of use, deforming a metal sheet held at its periphery between a hollow matrix and a blank holder, by pressing with the help of a die the sheet metal into the cavity of the matrix, a fluid under pressure, such as water, being introduced into the cavity to press the sheet metal against the die.
This forming process permits producing protrusions and complex shapes. Thus, the existence of a high counter-pressure, exerted by the fluid present in the matrix, permits strongly forcing the blank against the die and reproducing faithfully on the latter all the projections and complex shapes.
It moreover permits avoiding localized deformations which give rise to substantial thinning of the initial sheet metal and require the use of substantial local polishing operations. It therefore permits giving the best results from the limit curves of forming. Supplementally, with this process, there is practically no elastic spring back of the material.
Nevertheless, it is seen that, when a blank is formed by the hydroforming process, it tends nevertheless to become thin in the portions which correspond to the angles, to the bends or other changes in direction and for this reason, the elements thus formed have weakened resistance in these portions, which can however be strongly stressed portions in use.
To overcome this drawback, there can be provided reinforcements constituted by pieces of the same nature as the blank, which are cut out to the desired shape and placed in the locations it is desired to reinforce. These pieces can be first welded or disposed in a manner to bond to the blank by diffusion, prior to hydroforming, if this technique is employed. This manner of proceeding is suitable in simple cases, but for complicated pieces, it requires cutting out with precision and securing to suitable locations, a large number of reinforcements. A more precise study of the stresses often requires that the reinforcements have different thicknesses from each other, which complicates the work.
Another solution, proposed by French patent 2 647 373, consists in giving the blank a non-constant thickness such that the piece obtained after forming has at all points the desired thickness. Thus, before its deformation, the portions of the blank which have a greater thickness than the rest of the blank, constitute reserves of material which, during deformation, compensate the thinning of the blank at all portions where it would be observed.
Moreover, in the hydroforming process, the stamping force is substantially greater than the force strictly necessary for the deformation of the material. Thus, the movement of the stamping die supposes a force proportional to the cross-section and to the pressure. The total force to be used is the sum of the stamping force plus the force required to overcome the counter-pressure. The stamping force can become negligible compared to the counter-pressure force. It is thus necessary to use very powerful machines to be able to produce pieces by a hydroforming process.
To solve this problem, there has been proposed by French patent 2 723 867 a hydraulic stamping process for a metallic blank consisting in placing the blank on a matrix having a central chamber to permit stamping by means of a die penetrating the chamber of the matrix and acting on a first surface of the blank and by using the hydraulic counter-pressure provided on the second surface of the blank by a liquid disposed in the matrix chamber. According to this process, the volume in the chamber is constant or nearly constant. The die is fixed relative to a frame and it is the matrix which is movable in translation so that the die penetrates the matrix chamber. In the practice of this process, the bottom of the chamber is constituted by a fixed piston sliding axially in the matrix, this piston being of a cross-section equivalent to that of the die. This permits obtaining a constant volume in the central chamber of the matrix.
However, these various techniques do not eliminate for all that the so-called xe2x80x9csockxe2x80x9d effect inherent in the principle of hydroforming.
This effect manifests itself when the play between the die and the blank holders is great. It corresponds to the swallowing of material brought about by the counter-pressure prevailing in the cavity of the matrix. Thus, the counter-pressure prevailing in the cavity being high, the material is pressed back and enters into the space which exists between the blank holders and the die. It thus forms a fold of material that is in danger of being pinched during descent of the die and thus to be weakened. The resulting piece could ultimately break in line with this bend, under force imposed during its normal use.
The present invention seeks precisely to overcome this xe2x80x9csockxe2x80x9d effect by providing a hydroforming process avoiding the formation of folds between the blank holder and the die.
To this end, the invention has for its object a sheet metal hydroforming process, in which a blank is secured with the help of a blank holder on a matrix provided with a cavity for stamping said blank with a die entering the matrix, characterized in that during the stamping phase, the play between the die and the blank holder is confined within a range of values preventing between said die and the matrix, the formation of a fold, as a function of one end or the other of the following parameters: pressure of the fluid in the cavity of the matrix and position of the die relative to the matrix.
Preferably, the control of the play between the die and the blank holder is carried out by a modulation of the geometry of the portions facing said die and of said blank holder during stamping, said modulation taking place either during all the duration of stamping, or during an initial phase of said stamping, according to the establishment of a minimum permanent play.
Preferably, only one of the two elementsxe2x80x94die and blank holderxe2x80x94is provided with a portion of variable geometry.
The process is applicable to matrices whose cavity is connected or not to a source of fluid, preferably a liquid, for counter-pressure.
The invention also has for its object a device for practicing the above process, comprising a matrix with a cavity adapted to receive a stamping die for a metallic blank fixed on the matrix with the help of a blank holder, characterized in that it comprises means to modify the play between the die and the blank holder in the course of the stamping operation.